I've had beta7 installed for some time now and just experienced something similar last night...

I couldn't get ethernet to connect (my fault because of ip settings) so I plugged in the USB cable to change the IP settings. The player showed as available via USB, but was grayed out and sayed "no longer availabe"....

A simple reboot of the player fixed it all. Ask over in Bug Reports and you may get more expert advice than I can give! good luck!